From Bulbapedia, the community-driven Pokémon encyclopedia.
Soul Dew (Japanese: こころのしずく Soul Droplet) is a held item that raises the Special Attack and Special Defense of Latios or Latias by 50% when held.
Because of this large boost, it has been banned from official tournaments. If used in the Battle Tower, Battle Hall or Battle Subway, Soul Dew's effect is nullified.
In Pokémon Ruby, Sapphire, and Emerald, it is held by the wild Latios or Latias (depending on version) that resides on Hoenn's Southern Island.
In Pokémon Battle Revolution it may be purchased for 14,400 Poké Coupons from the Mystery Gift shop once the player has completed Stargazer Colosseum.
In Pokémon HeartGold and SoulSilver, if the player has received the Enigma Stone from an event, the player can take it to Steven Stone in the Pewter Museum of Science, who will have a researcher analyze it and finds out that it is actually Soul Dew. After he gives the player the Soul Dew, LatiosHG or LatiasSS will be waiting outside to battle the player.
In Pokémon Black 2 and White 2, it appears next to the player in Dreamyard after LatiosB2 or LatiasW2 is caught.
In the anime
sight-sharing with Latios
while in Soul Dew form after its sacrifice
In the anime, it appeared in the movie Pokémon Heroes, in which it is used to guard Johto's Alto Mare. It was given to an old couple in Alto Mare long ago after Latios and Latias saved the city. The Team Rocket duo Annie and Oakley were looking for Soul Dew as well as the two legendary guardians of the city, Latios and Latias. If it is attached to the Defense Mechanism of Alto Mare (DMA), anyone using the device can drown the city in water, block all exits in the city and awaken the Fossil Pokémon, Kabutops and Aerodactyl. It is implied that this Soul Dew contains the soul of a deceased Latios.